There are various shapes of projections that are used for projection welding. From the standpoint of increasing the welding strength, several requirements have to be satisfied with respect to the projection. In one respect, the projection should have a sharp tip. In addition, the projection should be able to withstand the pressing force imposed on the projection from the electrode without causing large deformation.
If the above-described two requirements are not satisfied, a sufficient welding strength will not be obtained. This is particularly so in the case where the welding length is long, such is the case with ring projection welding.
The projection is formed usually by pressing or forging. From the viewpoint of cost, pressing is preferable. Japanese Patent Publication HEI 2-280975 discloses forming the projection by pressing and then conducting projection welding by pressing the projection against another workpiece. FIG. 11 of the present application illustrates the formation of the projection by pressing while FIG. 12 depicts the way in which the projection welding is conducted by pressing the projection against another workpiece.
In the projection welding shown in FIGS. 11 and 12, since the projection 2 formed in one plate 1 is generally J-shaped, the projection 2 is deformed to a great extent when a pressing force acts on the projection during projection welding. Thus, the contact area of the projection 2 with another workpiece 3 is large. As a result, the electrical current density of the welding electrical current 4 is low, which lowers the amount of heat generated at the contact portion S. This causes insufficient melting of the contact portion and lowering the welding strength.